1. Field of the Invention
The present invention relates to an improvement of engagement between a pinion attached to an output shaft of an overhang type starter motor and a ring gear.
2. Discussion of Background
FIG. 4 is a cross-sectional view of an important portion of an output side of a conventional overhang type starter motor disclosed in, for instance, Japanese Unexamined Patent Publication No. 97576/1991.
In FIG. 4, reference numeral 1 designates an overrunning clutch in which a clutch outer is spline-engaged with a rotary output shaft. A torque produced in the clutch outer is transmitted to a clutch inner formed integrally with a pinion shaft 2 by means of a roller or the like. The pinion shaft 2 is supported so as to be slidable in the axial direction by means of a ball bearing 3 which is fitted to a front bracket 4. A pinion 5 is spline-engaged with a straight spline 2a formed in the pinion shaft 2 at a front side of the ball bearing 3. The overhang type starter motor is derived from a structure that the pinion 5 is located in front of a part for supporting the rotary shaft.
A pinion stopper 6 and a ring 7 are disposed to restrict a movement of the pinion 5 in a forward direction. The pinion 5 is urged toward the pinion stopper 6 by means of a spring 8.
Numeral 9 designates a plunger which constitutes a part of an electromagnetic switch. When the plunger 9 is attracted, a lever 10 is turned around a pivotal point in a counterclockwise direction to thereby urge the overrunning clutch 1 in the forward direction. Numeral 11 designates a ring gear provided in the side of an internal combustion engine.
A character X represents a clearance between the outer periphery 2b of the pinion shaft and the inner periphery 5a at a rear portion of the pinion, the clearance being usually set to be about 0.55 mm. A character Y represents a clearance between the outer periphery of the pinion 5 and the inner periphery of the front bracket 4, the clearance Y being usually set to be about 1.5 mm.
The operation of the conventional starter motor will be described.
When a switch coil (not shown) is excited whereby the plunger 9 of the electromagnetic switch is attracted, the lever 10 is turned in the counterclockwise direction and the overrunning clutch 1 is urged forwardly. Then, the pinion shaft 2 is slidably moved forwardly while it is supported by the bearing 3. The sliding movement of the pinion shaft 2 brings an end surface of the pinion 5 spline-engaged with the pinion shaft 2 into contact with the ring gear 11. Then, the pinion 5 is engaged with the ring gear 11 with deflection of the spring 8.
Thereafter, a switch contact (not shown) is closed to start the revolution of a motor shaft, and a rotating force of the motor shaft is transmitted to the overrunning clutch 1. The rotating force of the pinion shaft 2 which constitutes a part of the overrunning clutch 1 is transmitted to the pinion 5 through the serration of the spline 2a so that the ring gear 11 of the engine is rotated.
The engagement of the pinion 5 with the ring gear 11 is conducted by using an auxiliary rotation type intermeshing mechanism which is so operated that when the end surface of the pinion 5 becomes in contact with the end surface of the ring gear 11, a main contact of a switch (not shown) is not closed but a current flowing in an exciting coil of the switch is supplied to a motor (not shown), whereby the motor produces a torque which causes rotation by overcoming a friction between the end surfaces of the pinion 5 and the ring gear 11. Then, the pinion 5 is turned for one pitch by the torque, whereby the pinion 5 is engaged with the ring gear 11. When the pinion 5 is engaged with the ring gear 11 over the total face width, the main contact of the switch is closed to produce a full starting torque. Thus, the ring gear 11 is rotated.
In the conventional starter motor having the above-mentioned construction, the pinion 5 located on the pinion shaft 2 has the following problem. An inclination of the pinion 5 at the time of engagement with the ring gear 11 is mainly determined by the tooth crest, the bottom land or the tooth surface of the serration of the spline 2a, namely, the inclination of the pinion 5 is restricted only by a fitting portion in the serration of the spline 2a. Since the serration has a backlash (in particular, the serration is formed by rolling or cold forging whereby there is a limitation to improve precision of processing) and the dimension of the clearance X is relatively large as 0.55 mm, there is a tendency that the spring 8 is inclined when the pinion 5 is brought to contact with the end surface of the ring gear 11 (as shown in FIG. 5).
When the spring 8 is inclined, a point contact state between the pinion 5 and the ring gear 11 is created at their end surfaces. The portion having a point contact between the pinion 5 and the ring gear 11 is apt to cause a damage, and the frictional coefficient at the contacting point is apt to be large when the torque for turning the pinion 5 for one pitch is produced. Accordingly, it is difficult to obtain the one pitch rotation of the pinion and a fault of starting may possibly occur. Further, even after the pinion 5 was engaged with the ring gear 11, the transmission of the torque through the tooth surfaces of the pinion 5 is not stable due to looseness of the pinion 5, with the result of generation of noises and the wearing of the tooth surfaces of the pinion.
Since a load applied to the tooth surfaces of the pinion 5 is born by the serration portion of the pinion shaft 2 by means of the spline serration 2a, a span Li between the bearing 3 for supporting the pinion shaft 2 and a point under load is long as shown in FIG. 5, so that there is a problem from the viewpoint of the strength of the pinion shaft.
Further, the inclination of the pinion 5 might cause interference between the outer periphery of the pinion 5 and the inner peripheral portion 4a of the front bracket. Accordingly, it is necessary to set the value of the clearance Y to be large. When the clearance Y is made large, a dust-proof function decreases so that dust deposits on the outer circumferential surface of the pinion shaft 2. This might cause a fault of sliding movement of the pinion shaft 2.